Replacement surgery for artificial joints, in particular of the femoral component of hip joints, entails removal of the bone end-section. The end of the bone and the corresponding joint component are replaced with a prosthesis which must assume the load supporting function. Joint forces are transferred from the prosthesis to the bone. From the very early days of joint replacement till today the preferred mode for anchoring the prosthetic component in the tubular bone has been by means of a stem, i.e. a rod-shaped extension of the prosthesis properly adapted to the shape and size of the medullary cavity. Since the early sixties the standard procedure has included the use of so-called bone cement. Bone cement is a room temperature curing acrylate (usually in the form of a powder component consisting of polymethylmethacrylate with a catalyst and a liquid component consisting of methylmethacrylate with an accelerator, the two components polymerizing upon mixing with each other) with additives such as zirconium oxide (for obtaining radiographic density) and antibiotics. When cured, bone cement forms a relatively soft (about five to ten times softer than cortical bone) mantle between the prosthesis stem and the bone. Stresses induced in this cement mantle by load transfer from the stem to the bone depend on the compliances of the stem and of the bone.
In general, a stiffer stem will transfer loads closer to its tip, i.e. more distally in the case of the femoral component of the hip joint; a more compliant stem will transfer loads closer to the neck, i.e. more proximally. Gradual change of the stem cross-section (smaller cross-section distally) results in more gradual load transfer. This works well for the distal part of the stem, but it moves the load transfer more proximally, where the cross-section of the stem cannot be sufficiently reduced since it must support the joint loads transferred to it via the neck of the prosthesis. In addition, the bone tube is opened at this side and thus rendered even softer. To achieve a truly uniform load transfer the stem should reduce its stiffness all the way to zero at its proximal end.